Two legged robot designs are generally inspired by human anatomy. In robotics research, humanoid robots have the highest budget due to the high interest of people on humanoids. The purpose of designing humanoid robots is to create robots that can use human tools and work on environments designed for humans. Balance is an important design consideration while designing bipedal locomotion methods. For that reason these robots have to check their balance while moving. Bipedal robots can have additional mechanisms to keep in balance while moving. Because of the balance problem, bipedal locomotion methods are controlled using complex algorithms.
The most basic method implemented on bipedal robots is passive dynamic walking which is very similar to wheeled locomotion. In this system, legs rotate like wheels. If the weight is distributed evenly each legs act like an inverted pendulum and the robot rotates between steps. To enhance the design, a knee can be added to lift the legs up. To add more detail to the leg, an ankle can be added for a spring effect to return the energy lost to the system while landing on the ground.